[Help setting margins and bleed, Affinity Publisher]

I am new to Affinity Publisher and would like help with setting up margins and bleed.

My document will go to a commercial printer.

I want to end up with an A5 size sheet with an invisible (on the printed version) margin 4mm from all the edges that prevents text and images etc from encroaching the border area.

My printer requires a 3mm bleed all round.

I can set the paper size 6mm larger in both directions to allow for the 3mm bleed, but how do I set up the margins?

The coloured background needs to extend to the edges of the larger sheet.

I wish to export as a pdf and need to make the printed pdf appear in the centre of the A4 paper in my printer so that I can check the measurements.

Thanks in advance

Geoff